Holographic stereogram is prepared, e.g., by recording, in succession, onto single recording medium for hologram as strip-shaped or dot-shaped element hologram, a large number of pictures, as original picture, which are obtained by successively photographing object from different points of observation.
For example, in preparing holographic stereogram having parallax information only in the lateral direction, such an approach is employed as shown in FIG. 1 to first photograph in succession object 100 from different points of observation in the lateral direction to thereby obtain parallax picture train 101 consisting of plural pictures having parallax information in the lateral direction. It is to be noted that it is sufficient that such parallax picture train 101 is not picture obtained by actually photographing object, but may be, e.g., CAD (Computer Aided Design) picture or CG (Computer Graphics) picture generated by computer etc. Further, respective pictures 102 constituting this parallax picture train 101 are recorded in succession onto a recording medium 103 for hologram as strip-shaped element hologram in such a manner that they are successive in the lateral direction. Thus, holographic stereogram having parallax information in the lateral direction can be obtained.
In this holographic stereogram, since information of plural pictures 102 obtained by successively photographing object 100 from different points of observation in the lateral direction are recorded in succession in such a manner that they are successive in the lateral direction as strip-shaped element hologram, when observer looks at this holographic stereogram by the both eyes, two-dimensional picture images respectively imaged on his left and right eyes are different. Thus, the observer feels parallax so that three-dimensional picture image is reproduced.
Meanwhile, element holograms of the above-mentioned holographic stereogram are recorded onto recording medium for hologram using photosensitive material as recording material in a manner stated below. Namely, in recording element holograms onto the recording medium for hologram, laser beams of good coherence are branched, and one branched laser beam is irradiated in a manner perpendicular to one surface of the recording medium for hologram as projected image (object light) which has been configured to undergo two-dimensional picture modulation by picture display means, e.g., liquid crystal panel, etc. Further, the other branched laser beam is irradiated at a predetermined angle onto the other surface of the recording medium for hologram as reference light. Thus, interference patterns are formed as a change of refractive index or transmission factor at the photosensitive material of the recording medium for hologram so that element holograms are recorded.
As a recording medium for a hologram, there may be used, e.g., a recording medium of the type in which a hologram recording layer consisting of photosensitive material such as photo-polymer, etc. is configured to be put between the protective layer and the base. The object light and the reference light are irradiated in the state of linear polarization from one surface and the other surface of the hologram recording layer to form interference patterns to record element holograms. It is to be noted that the photosensitive material of the hologram recording layer is not limited to photo-polymer, but there may be used other photosensitive material, e.g., silver salt material or gelatin bichromate, etc.
As the protective layer and the base of such recording medium for hologram, the so-called optical material such as optically transparent resin or glass, etc. is used. Further, as such protective layer and base, there is exemplified, e.g., transparent resin plate of such as polycarbonate, polyolefin, PMMA (polymethyl methacrylate), etc. as described in the Japanese Patent Publication No. H6-214117. Furthermore, there are exemplified PMMA (polymethyl methacrylate), polycarbonate, polyolefin, diethylene glycol bisallyl carbonate, polystyrene, hard polyvinyl chloride, methylmethacrylate-styrene copolymer resin, styrene-acrylonitrile copolymer resin, and poly (4-methylpentene-1), etc., as described in the Japanese Patent Publication No. H10-119163. Further, Japanese Patent Publication No. 11-338336 discloses as examples silicon compositions, etc. of condensation reactive type, addition reactive type, non-solvent type, ultraviolet hardening type and electron beam hardening type, etc.
In addition, as structure of hologram recording medium including hologram recording layer consisting of photo-polymer layer serving as photosensitive material as described above, there are, as the structure already announced, “Stephen A. Zager and Andrew M. Weber, “Display holograms in Du Pont's Ommidex films”, Proc. of SPIE, Vol. 1461 (1991) pages 58–67 [DuPont]”, “T. J. Trout, W. J. Gambogi and S. H. Stevenson, “Photopolymer Materials for Color Holography”, Proc. SPIE, Vol. 2577 (1995) pages 94–105”, “Sylvia H. Stevenson, “DuPont multicolor holographic recording films”, Proc. of SPIE, Vol. 3011 (1997)”, “Masami Kawabata, Akihiko Sato, Iwao Sumiyosi and Toshihiro Kubota, “Photopolymer system and its application to a color hologram” Applied Optics, Vol. 33, No. 11 (Apr. 10, 1994) pages 2152–2156”, etc.
However, optical materials having small difference between refractive indexes of double refraction are expensive. Further, according as double refraction becomes smaller or lesser, materials are limited, resulting in higher cost.
As described above, object light and reference light are incident on the hologram recording layer as linearly polarized light to form interference patterns. However, when there is double refraction characteristic (birefringence) in the base or the protective layer, plane of polarization becomes oblique or changes into elliptically polarized light when such light is passed (transmitted) through the base or the protective layer. For this reason, contrast of interference pattern within the hologram recording layer is lowered.
As a method of solving such a phenomenon, there is mentioned a method of reducing retardation which is one of index values of double refraction, i.e., reducing difference between refractive index in the direction in parallel to the optical axis and refractive index in the direction perpendicular to the optical axis as described in the Japanese Patent Publication No. H7-114329, for example.
However, optical materials having small difference between refractive indexes of doble refraction are expensive.; Further, according as double refraction becomes smaller or lesser, materials are limited, resulting in higher cost.
In addition, in development and fixing of photosensitive material such as silver salt material, gelatin bichromate, photo-polymer, etc. used in the hologram recording layer, development process and fixing process are carried out by acid/alkali, ultraviolet ray/visible ray/infrared ray, heat of high temperature or combination thereof, etc.
Under such circumstances, optical materials used in the protective layer and the base are also required to have various resistances or tolerances, e.g., chemicals resisting property such as acid resistance or alkali resistance, etc., water proof property such as moisture proof property or swelling proof property, etc., light proof property such as yellowing proof property, etc., weathering resistance property such as heat resistance property or yellowing resistance property, etc.
Accordingly, optical materials having less double refraction and provided with the above-mentioned resistances are limited and are therefore very expensive.